Enrichment Mechanism of the Organic-Rich Shale in Ediacaran Doushantuo Formation in Western Sichuan Basin, South China

The Ediacaran Doushantuo Formation in South China has been increasingly recognized as a significant potential source rock for hydrocarbon reservoirs, particularly in relation to the overlying Dengying Formation. This study conducts a comprehensive geochemical analysis of the organic-rich shale from the Ebian-Xianfeng section within the Sichuan Basin, aimed at elucidating the mechanisms of organic matter enrichment. The shale in Member II is characterized by high total organic carbon (TOC) content, ranging from 1.03 to 4.06 wt%. Geochemical proxies, including the chemical index of alteration (CIA) and redox-sensitive trace elements (Mo, U, and V), exhibit an upward increase, indicative of a paleoenvironmental transition towards more humid and anoxic conditions. The positive correlation between enrichment factors (Mo-EF, U-EF, and V-EF) and TOC content suggests that redox conditions were a primary control on organic matter preservation. Minimal evidence of hydrothermal influence indicates that organic matter remained largely unaltered postdeposition. The original findings suggest that the period of organic-rich shale deposition was likely a relatively warm and humid time, during which enhanced weathering input more nutrients, thereby increasing productivity and leading to the generation and sedimentation of more organic matter into the shale. Moreover, the cooling period following the warm phase may have intensified the oxidation of dissolved organic carbon (DOC) in the water column and porewater. This process consumed oxygen and sulfate, thereby creating anoxic conditions that were favorable for the preservation of organic matter. The geochemical signatures further suggest that the organic-rich shale deposited in the intrashelf basin may not be as rich in organic matter as those in deepwater basins and slopes, but it still holds certain potential for oil and gas. This study emphasizes the significance of climatic fluctuations, redox dynamics, and hydrothermal stability in the formation of organic-rich shale within the Doushantuo Formation, thereby contributing to the hydrocarbon potential of the Sichuan Basin.